Data storage devices such as hard disk drives, optical drives, and the like typically utilize rotatable storage media from which data is read and/or to which data is written. A hard disk drive, for example, may be constructed using one or more disks of magnetic storage media coupled to a rotatable spindle supported by one or more bearings. An armature of a motor is coupled to the spindle is used to rotate the disk(s), while one or more actuators having read/write heads disposed at their ends are swept back and forth across the radii of the disks to read data from and/or write data to the disks. Similarly, an optical drive such as a CD-ROM or a DVD drive relies on a spinning optical disk and a radially-movable read/write head.
Regardless of the storage medium used, data is typically arranged into concentric tracks on the medium, and precise positioning of the actuator, and thus of the read/write head, by a servo system is typically required to ensure that the read/write head is positioned over the correct track on the storage medium.
Today there is an ever increasing demand on data storage capacity and access speed. These demands are fueled by the development of new processors that run faster and faster, executing more and more instructions per second. The programs containing these instructions have also become more voluminous, along with the data accessed by these programs. Consequently, data storage devices must also offer reduced access speed along with additional storage capacity.
One primary manner of increasing both storage capacity and access speed is through increasing track density, which is typically tracked in terms of tracks per inch (TPI). Increased track density allows more data to be stored on a given disk size. In addition, access time is also reduced since read/write heads do not need to be moved as far between tracks to read or write data.
As track density increases, however, greater positioning accuracy of an actuator is likewise required. Furthermore, vibrational effects such as mechanical vibrations and the like have a comparatively greater effect on alignment of a read/write due to the reduced spacing between adjacent tracks on a disk. As a result, significant development efforts have been directed to improving data storage device servo systems that control read/write head registration.
One particular area of concern is related to narrowband mechanical excitations of an actuator in a hard disk drive, which often result in increased nonrepeatable track misregistration (TMR), and consequently reduced disk drive performance. One such narrowband mechanical excitation stems relates to nonrepeatable runout (NRRO) effects from defects in the bearings used to rotatably support a drive spindle, e.g., where the bearings are Brinelled due to dents in the bearings' raceways. Such defects tend to generate a spindle excitation that is fixed in frequency, but still nonrepeatable due to a non-repeatable phase.
Due to mechanical coupling within the disk drive, NRRO excitations are ultimately detected by the drive's actuator servo controller, which adds spurious frequency components to the controller's control loop, and hampers the controller's ability to accurately position the actuator for proper track registration. As a result, the feedback to the controller, typically in the form of a Position Error Signal (PES), is exaggerated, which can result in misreads and write inhibits, and ultimately in reduced drive performance.
Furthermore, in some drive designs, the disturbance frequency resulting from NRRO mechanical excitation is near the open loop zero crossover frequency of a servo controller. As a result, the excitation is actually amplified by the servo controller, which further exaggerates the PES and reduces drive performance.
Another concern raised by NRRO and other mechanical excitations is an inability in conventional data storage devices to detect such excitations before they have an adverse impact on drive performance. Many hard disk drives, for example, include predictive failure analysis (PFA) functionality that attempts to detect potential defects in a drive prior to drive failure, so that a user can be notified of the defects in time to transfer any critical data to another data storage device and replace the failing drive. In fact, a number of hard disk drive manufacturers support a common reporting standard referred to as Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.). While such functionality is often able to detect a number of potential defects, such functionality is not capable of detecting defects that cause NRRO and related mechanical excitations. As a result, progressive NRRO and other related mechanical excitations can ultimately be the cause of unexpected drive failures, and consequent loss of drive data.
Therefore, a significant need exists in the art for a manner of detecting and/or correcting for defects in a data storage device resulting from NRRO and other mechanical excitations.